1. Field of the Invention
The present invention relates to a color image forming apparatus preferably applied to a tandem type color printer, a copying machine or to a combination machine thereof provided with an intermediate transfer belt and a plurality of photoconductor drums.
2. Background Technology
In recent years, a tandem type color printer and a copying machine as well as the combination thereof have come into frequent use. The image forming apparatus of this type is provided with an exposure section, a development apparatus, a photoconductor drum, an intermediate transfer belt and a fixing apparatus for each color of the yellow (Y), magenta (M), cyan (C) and black (BK).
For example, in the Y-color exposure section, a light beam scanning is applied by a polygon mirror based on the color image information, and an electrostatic latent image is formed on a photoconductor drum. In the development apparatus, a Y-color toner member is attached to the electrostatic latent image formed on the photoconductor drum, whereby a color toner image is formed. The photoconductor drum allows the toner image to be transferred onto the intermediate transfer belt. The same procedure is applied to other M, C and BK colors. The color toner image transferred onto the intermediate transfer belt is fixed by the fixing apparatus after having been transferred onto paper.
The control system for forming a color image on a predetermined sheet of paper is provided with a signal creating section and control section. The signal creating section generates an image valid area signal for setting the start position and width of an image forming area in the sub-scanning direction for each color. The sub-scanning direction here refers to the direction orthogonal to the main scanning direction in which a light beam is applied through a polygon mirror. The control section provides color imaging control on a predetermined side of paper, based on the image top signal for controlling the writing of an image on the photoconductor drums, the image valid area signal for each color created by the signal creating section, and the main scanning direction reference signal (hereinafter referred to as “index signal”) for the rotation speed control and face phase control of the polygon mirror.
The reference index signal corresponds to the horizontal sync signal for regulating the writing of an image in the horizontal direction of paper (main scanning direction). The image valid area signal corresponds to the vertical sync signal for regulating the writing of an image in the vertical direction of paper (sub-scanning direction). The image valid area signal is created based on the reference index signal. For example, a counter is provided for each color to count the pulse of the reference index signal in such a way that the image valid area signal will be outputted.
Japanese Unexamined Laid-Open Patent Publication No. 11-143163 (hereinafter referred as Patent Document 1) discloses a color printer, with reference to the color image forming apparatus. In the color printer of this Patent Document 1, the horizontal sync signal is outputted while the vertical sync signal for printing on the first page of paper is outputted, and video data is outputted in response to this horizontal sync signal. This color printer contains a counter for each of Y, M, C and BK colors. Using the software counter function, the CPU generates data in units of several bits for selecting the vertical sync signal, and the selection signal obtained by decoding this selection data is outputted to the counter for each color.
In response to the BK-color horizontal sync signal, the counter for BK color counts the selection signal and outputs the BK-color vertical sync signal. In response to the Y-color horizontal sync signal, the counter for Y color counts the selection signal and outputs the Y-color vertical sync signal. In response to the M-color horizontal sync signal, the counter for M color counts the selection signal and outputs the M-color vertical sync signal. In response to the C-color horizontal sync signal, the counter for C color counts the selection signal and outputs the C-color vertical sync signal.
As described above, a single counter is provided for each color, and software counter allows printing to be performed in all colors. This arrangement minimizes possible increase of the apparatus costs, and ensures higher printing speed.
Japanese Unexamined Laid-Open Patent Publication No. 2004-009349 (hereinafter referred as Patent Document 2) discloses an image forming apparatus for forming a multi-color image by superimposition of a plurality of single-color images. This image forming apparatus is a tandem apparatus capable of switching the printing mode, and is equipped with a scanning start signal generating section and an image formation timing generating section. For the colors used in common with both the all-color mode including all different colors and the subtractive color mode wherein colors are subtracted from those in the all-color mode, the scanning start signal generating section generates the scanning start signal of a light beam used in the subtractive color mode. The image formation timing generating section generates a timing signal for each color in each color mode, in response to the scanning start signal generated by the scanning start signal generating section.
In this case, the image formation timing generating section is equipped with a single clock counter. In response to the rise of the scanning start signal of the light beam used in the subtractive color mode, counting operation is reset. Then the video clock is inputted to start a new counting operation. The video clock is outputted to the comparator circuit for generating line sync signal for each of the BK, Y, M and C color. When such an image formation timing generating section has been arranged, the image write position of each color can be correctly controlled, and reduced production cost of the overall apparatus is ensured, even when a scanning start signal is generated by receiving light from the light source of one particular color.
A conventional tandem color image forming apparatus involves the following problems:
(i). The color printer disclosed in the Patent Document 1 contains one counter each of the Y, M, C and BK colors. When a certain job is given, this printer uses the software counter function of the CPU to output the vertical sync signal (image valid area signal) for the BK, Y, M and C colors from each counter.
When the next job is given, the image valid area signal is set for each color after the image top signal of that job has started up. Accordingly, the next image top signal does not rise until the counting operation of the final color terminates, and therefore, the image valid area signal for the next job cannot be set during the image formation of the final color.
This conventional arrangement depends heavily on software control, and makes it difficult to simplify the image writing unit structure and to reduce the CPU operation load. Not only that, this arrangement makes it difficult to increase the speed in color image formation.
(ii). When a color image is to be formed on the front and rear faces of paper, a single clock counter as shown in the Patent Document 2 is provided. The counting operation is reset and in response to the rise of the scanning start signal, and a new counting operation is started based on the video clock. When this function is employed, the counter source as a reference cannot be as desired. Accordingly, if the image formation size is reduced on the front and rear faces of paper, continuous image formation (smooth linear speed switching operation) on the front and rear faces will be difficult. This makes it difficult to increase the speed in color image formation, similarly to the case of the aforementioned (i).